Amorphous silica particles have a variety of uses. For example, they can be used as fluid thickeners, thermal insulation, anti-caking agents, molecular sieve materials, and reinforcing fillers for silicone and rubber. These diverse uses often demand diverse physical properties of the silica, such as pore size, pore size distribution, particle size, surface area, density, surface properties, and the like.
For example, it is known that when silica particles are to be used as fillers in silicone rubber formulations, they become increasingly reinforcing as their sizes diminish. It is further known that the undesirable phenomenon of "crepe hardening" in filler applications can be avoided if the silica particles have been treated to make their surfaces hydrophobic. This process is often referred to as "hydrophobing" or "treating" the silica, and the organic compounds used for this process are called "hydrophobing agents".
Silica that is suitable for use as a silicone rubber filler can be prepared by hydrophobing silica from a number of sources. The most common source of silica is that obtained via neutralization of a solution of sodium silicate. Silica prepared in this manner, however, is often contaminated by a range of impurities, and it is difficult to control the physical properties of the formed particles.
Silica also may be formed by burning SiCl.sub.4 in the presence of methane or hydrogen to produce SiO.sub.2, H.sub.2 O, and HCl. Silica produced in this fashion, called pyrogenic or fumed silica, is quite pure (&gt;99% SiO.sub.2), and the particles are of suitable dimensions for use in filler applications (e.g., 2-3 .mu.m in diameter). However, the pyrogenic process is expensive to perform, and a hydrophobing step must be executed separately to obtain treated silica.
An inexpensive alternative to pyrolysis is the hydrolysis of a halosilane in solution. However, these reactions are generally so rapid that it is difficult to control the physical properties (e.g., particle size, density, surface area) of the products.
Irrespective of its method of preparation, silica may be treated in a number of ways. For example, U.S. Pat. No. 3,122,520 discloses a general method for hydrophobing silica. In this procedure, the silica is first suspended in water to form a hydrosol. The hydrosol is then acidified, heated, and treated with an organosilicon compound to cap the surface silanol groups. The hydrophobed silica is finally partitioned into an organic solvent, which is boiled away to provide the treated product.
While this method provides treated silica particles suitable for use as silicone rubber fillers, it requires the prior formation of a silica hydrosol, which is difficult to prepare from fumed silica, and the use and subsequent removal of organic solvents.
An alternative method for preparing treated silica suitable for use as a silicone rubber filler is described in U.S. Pat. No. 4,344,800 (Lutz). This patent teaches the use of a silicon alkoxide as a starting material instead of a silica hydrosol. The silicon alkoxide is mixed with water, an alcohol, and a hydrophobing agent, aged for a period of time, and then filtered and/or oven-dried to provide the filler material. Although this method generates treated silica from a silica precursor in a single process, it employs expensive silicon alkoxides, and requires the use and subsequent removal of an organic solvent.
There exists a need for a simple method of preparing treated silica that uses inexpensive processes and starting materials, does not require the use of an organic solvent, has the flexibility to provide silica particles in a range of sizes, surface areas, densities, etc., and provides those particles in high purity. The present invention provides such a method.